GSM covers making phone calls, sending SMS text messages, and access to Internet via GPRS. A SIM (Subscriber Identity Module) card is given along the contract with an operator, and the customer inserts the card in the phone to authorize it for use other than emergency dialing. The card is plastic (approximately 15mm*23mm*1mm) with 8 electrical contacts on. Its main purpose is identifying for the phone, the relationship between the contractor and the operator. This includes information about the mobile phone number, the home network, sms center phone number etc. The user (i.e. phone) usually has some amount of storage available on the card for a simple phonebook. SIM cards are standard ISO/IEC 7816 smart cards with a special SIM application on it.

The Neo1973 phone should work with any operator that has a GSM network (see Neo1973 compatible cellphone providers). However, in the current phone versions, the 850 MHz frequency needed in some areas in the Americas doesn't work. The other three frequencies and thus most areas in the world are supported: 900, 1800, and 1900 MHz. Some of the US and some other parts of the world completely lack GSM coverage.

According to User:roh on #openmoko, the 850 band doesn't work with the current GTA01 GSM modem, because RF filter was set to 900 Mhz at GTA01. This is being worked on. This means that if you're in the Americas, there is a good chance the phone will simply not get a signal, unless you check that your provider has 1900 MHz coverage in your area. (For more on the frequencies, see GSM frequency usage in the Americas.) GTA02 has 2 versions, 850/1800/1900 Mhz and 900/1800/1900 Mhz version.